len(). Should return
the length of the object, an integer >= 0. Also, an object
whose __len__() method returns 0 is considered to be false in a
Boolean context.
self[key]. Note that the
special interpretation of negative keys (if the class wishes to
emulate a sequence type) is up to the __getitem__ method.
self[key]. Same note as for
__getitem__.
self[key]. Same note as for
__getitem__.